Surveillance of Waterborne Disease Outbreaks Associated with Drinking Water — United States, 2015–2020

Surveillance Summaries / March 14, 2024 / 73(1);1–23

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Jasen M. Kunz, MPH1; Hannah Lawinger, MPH1; Shanna Miko, DNP1; Megan Gerdes, MPH2; Muhammad Thuneibat, MPH2; Elizabeth Hannapel, MPH3; Virginia A. Roberts, MSPH1 (View author affiliations)

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Abstract

Problem/Condition: Public health agencies in U.S. states, territories, and freely associated states investigate and voluntarily report waterborne disease outbreaks to CDC through the National Outbreak Reporting System (NORS). This report summarizes NORS drinking water outbreak epidemiologic, laboratory, and environmental data, including data for both public and private drinking water systems. The report presents outbreak-contributing factors (i.e., practices and factors that lead to outbreaks) and, for the first time, categorizes outbreaks as biofilm pathogen or enteric illness associated.

Period Covered: 2015–2020.

Description of System: CDC launched NORS in 2009 as a web-based platform into which public health departments voluntarily enter outbreak information. Through NORS, CDC collects reports of enteric disease outbreaks caused by bacterial, viral, parasitic, chemical, toxin, and unknown agents as well as foodborne and waterborne outbreaks of nonenteric disease. Data provided by NORS users, when known, for drinking water outbreaks include 1) the number of cases, hospitalizations, and deaths; 2) the etiologic agent (confirmed or suspected); 3) the implicated type of water system (e.g., community or individual or private); 4) the setting of exposure (e.g., hospital or health care facility; hotel, motel, lodge, or inn; or private residence); and 5) relevant epidemiologic and environmental data needed to describe the outbreak and characterize contributing factors.

Results: During 2015–2020, public health officials from 28 states voluntarily reported 214 outbreaks associated with drinking water and 454 contributing factor types. The reported etiologies included 187 (87%) biofilm associated, 24 (11%) enteric illness associated, two (1%) unknown, and one (<1%) chemical or toxin. A total of 172 (80%) outbreaks were linked to water from public water systems, 22 (10%) to unknown water systems, 17 (8%) to individual or private systems, and two (0.9%) to other systems; one (0.5%) system type was not reported. Drinking water-associated outbreaks resulted in at least 2,140 cases of illness, 563 hospitalizations (26% of cases), and 88 deaths (4% of cases). Individual or private water systems were implicated in 944 (43%) cases, 52 (9%) hospitalizations, and 14 (16%) deaths.

Enteric illness-associated pathogens were implicated in 1,299 (61%) of all illnesses, and 10 (2%) hospitalizations. No deaths were reported. Among these illnesses, three pathogens (norovirus, Shigella, and Campylobacter) or multiple etiologies including these pathogens resulted in 1,225 (94%) cases. The drinking water source was identified most often (n = 34; 7%) as the contributing factor in enteric disease outbreaks. When water source (e.g., groundwater) was known (n = 14), wells were identified in 13 (93%) of enteric disease outbreaks.

Most biofilm-related outbreak reports implicated Legionella (n = 184; 98%); two nontuberculous mycobacteria (NTM) (1%) and one Pseudomonas (0.5%) outbreaks comprised the remaining. Legionella-associated outbreaks generally increased over the study period (14 in 2015, 31 in 2016, 30 in 2017, 34 in 2018, 33 in 2019, and 18 in 2020). The Legionella-associated outbreaks resulted in 786 (37%) of all illnesses, 544 (97%) hospitalizations, and 86 (98%) of all deaths. Legionella also was the outbreak etiology in 160 (92%) public water system outbreaks. Outbreak reports cited the premise or point of use location most frequently as the contributing factor for Legionella and other biofilm-associated pathogen outbreaks (n = 287; 63%). Legionella was reported to NORS in 2015 and 2019 as the cause of three outbreaks in private residences (2).

Interpretation: The observed range of biofilm and enteric drinking water pathogen contributing factors illustrate the complexity of drinking water-related disease prevention and the need for water source-to-tap prevention strategies. Legionella-associated outbreaks have increased in number over time and were the leading cause of reported drinking water outbreaks, including hospitalizations and deaths. Enteric illness outbreaks primarily linked to wells represented approximately half the cases during this reporting period. This report enhances CDC efforts to estimate the U.S. illness and health care cost impacts of waterborne disease, which revealed that biofilm-related pathogens, NTM, and Legionella have emerged as the predominant causes of hospitalizations and deaths from waterborne- and drinking water-associated disease.

Public Health Action: Public health departments, regulators, and drinking water partners can use these findings to identify emerging waterborne disease threats, guide outbreak response and prevention programs, and support drinking water regulatory efforts.

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Introduction

Access to and provision of safe water in the United States is critical to protecting public health (1). Disruptions to water service caused by drinking water contamination can negatively impact public health and erode public trust in drinking water quality. Each year in the United States, waterborne pathogens cause an estimated 7.15 million illnesses, 118,000 hospitalizations, and 6,630 deaths, resulting in $3.33 billion in direct health care costs (2). Drinking water exposures are associated with 40% of hospitalizations and 50% of deaths and are primarily linked with biofilm pathogens such as Legionella and nontuberculous mycobacteria (NTM), costing the United States $1.39 billion annually (3). Biofilms are microbial communities that attach to moist surfaces (e.g., water pipes) and provide protection and nutrients for many different types of pathogens, including Legionella and NTM (3,4). Biofilm can grow when water becomes stagnant or disinfectant residuals are depleted, resulting in pathogen growth (3). Furthermore, biofilm pathogens are difficult to control because of their resistance to water treatment processes (e.g., disinfection) (3). Exposure to biofilm pathogens can occur through contact with, ingestion of, or aerosol inhalation of contaminated water from different fixtures (e.g., showerheads) and devices (e.g., humidifiers) (3).

Public health surveillance and other prevention programs support water treatment, regulations, and building or household water management practices in reducing waterborne diseases. Public health agencies in the United States, District of Columbia, Federated States of Micronesia, Guam, Marshall Islands, Northern Mariana Islands, Palau, Puerto Rico, and U.S. Virgin Islands investigate and can voluntarily report waterborne disease outbreaks to CDC through the National Outbreak Reporting System (NORS) (https://www.cdc.gov/nors/about.html).

This report summarizes data on drinking water-associated outbreaks reported to NORS during 2015–2020. Drinking water, also called tap or potable water, includes water collected, treated, stored, or distributed in public and individual water systems or commercially bottled and distributed for individual use. Drinking water is used for consumption and other domestic uses (e.g., drinking, bathing, showering, handwashing, food preparation, dishwashing, and maintaining oral hygiene). This report summarizes outbreak contributing factors (i.e., practices and factors that lead to outbreaks) and, for the first time, categorizes outbreaks as biofilm pathogen or enteric illness associated (5–9). Public health departments, regulators, and drinking water partners can use the findings in this report to guide outbreak response and prevention programs and drinking water regulatory efforts.

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Methods

Data Source

CDC’s Waterborne Disease and Outbreak Surveillance System began in 1971, with reporting via paper forms through 2008. CDC launched NORS (https://www.cdc.gov/nors/about.html) in 2009 as a web-based platform for state, local, and territorial health departments to enter reports of all waterborne and foodborne disease outbreaks and all enteric disease outbreaks resulting from transmission by contact with contaminated environmental sources, infected persons or animals, or unknown modes. Most outbreaks in the United States are investigated by state, local, and territorial health departments. Outbreak information is then voluntarily reported to CDC by the public health agency that conducted the investigation. CDC might be involved in outbreak investigations that involve more than one state, are particularly large, or for which the state or local health department requests assistance.

Waterborne Outbreak Definitions and Specifications

NORS users enter a confirmed or suspected etiology, if known, including species, serotype, or other characteristics. Etiologies for reported drinking water outbreaks in NORS can include infectious (e.g., Campylobacter, Cryptosporidium, Giardia, and Legionella) and noninfectious (e.g., copper and nonbacterial toxins) agents.

For NORS reporting, an outbreak is defined as two or more cases of similar illness associated with a common exposure. Outbreaks reported to NORS must include two or more cases linked epidemiologically by time, location of water exposure, and illness characteristics; the epidemiologic evidence must implicate water exposure as the probable source of illness for an event to be defined as a waterborne disease outbreak. Premise plumbing refers to a building’s hot- and cold-water piping system. For this analysis, the premise plumbing pathogens NTM, Pseudomonas, and Legionella were defined as biofilm-associated pathogens (2,10). In addition, consistent with the literature, Campylobacter, Cryptosporidium, Giardia, norovirus, and Shigella were defined as infectious enteric pathogens (2,11). Outbreaks of unknown etiology and noninfectious illness (e.g., chemical or toxin), were classified in their own categories.

Community and noncommunity water systems are defined by the Environmental Protection Agency as public water systems that have ≥15 service connections or serve an average of ≥25 residents for ≥60 days per year. A community water system serves year-round residents of a community, subdivision, or mobile home park. A noncommunity water system serves an institution, industry, camp, park, hotel, or business and can be nontransient or transient. Nontransient systems serve ≥25 of the same persons for ≥6 months of the year but not year-round (e.g., factories and schools), whereas transient systems provide water to places in which persons do not remain for long periods (e.g., restaurants, highway rest stations, and parks). Individual water systems are small systems not owned or operated by a water utility that have <15 connections or serve <25 persons (12).

NORS encourages users to indicate, when known, the water source and water source description for drinking water outbreaks. Water sources for drinking water outbreaks listed in NORS include groundwater, surface water, groundwater under the influence of surface water, other, and unknown. Water source descriptions listed in NORS include lake or reservoir, ocean, pond, river or stream, spring, well, other, and unknown.

NORS defines water treatment as the treatment usually provided before water use or water consumption, regardless of whether these treatments were operating correctly at or just before the time of the outbreak. Possible water treatment methods listed for the drinking water systems in NORS include disinfection, filtration, coagulation, flocculation, no treatment, other, and unknown.

NORS includes options to indicate where the exposure to water occurred. Settings of exposure for drinking water outbreaks listed in NORS include apartment or condominium; assisted living or rehabilitation facility, camp or cabin setting; community or municipality; hospital or health care facility; hotel, motel, lodge, or inn; long-term care facility; mobile home park; private residence; resort, restaurant or cafeteria; school, college, or university; subdivision or neighborhood; and several other types of settings.

NORS encourages users to indicate which, if any, contributing factors led to the outbreak. Users can select contributing factors from a list related to drinking water outbreaks or enter their own factors. Contributing factors for this reporting period included documented or observed (if information is gathered during document reviews, direct observations or interviews) or suspected (if factors that might have occurred but for which no documentation or observable evidence is available). Contributing factors were categorized by factor types including source (water quality was affected by a problem occurring with the source water), treatment (water quality was affected by a problem occurring with water treatment), distribution (water quality was affected by a problem within the distribution system before entry into a building or house), and premise or point of use (water quality was affected by a problem after the water meter or outside the jurisdiction of the public water utility).

Data Analysis

CDC analyzed outbreaks reported in NORS as of October 18, 2022, (via CDC 52.12 Form) to provide information about drinking water-associated waterborne disease outbreaks in the United States in which the first illness occurred during 2015–2020. For each outbreak, NORS users provided data (when known) about: 1) number of cases, hospitalizations, and deaths; 2) etiologic agent (confirmed or suspected); 3) implicated water system and treatment method; 4) setting of exposure (e.g., hospital or health care facility; hotel, motel, lodge, or inn; and private residence); and 5) relevant epidemiologic and environmental data needed to understand the outbreak occurrences and contributing factor classification.

CDC calculated descriptive statistics on characteristics of reported drinking water outbreaks. Data cleaning, management, and analysis were conducted using SAS (version 9.4; SAS Institute) and Microsoft Excel for Microsoft 365 Microsoft Office (version 2022; Microsoft Corporation). The analysis included both confirmed and suspected etiologies. Outbreaks with multiple etiologies were classified and analyzed as one outbreak.

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Results

All Outbreaks

Public health officials from 28 states reported 214 outbreaks associated with drinking water during the surveillance period (Tables 1, 2, 3, 4, 5, and 6) (Figure 1). Reported outbreaks included 187 biofilm-associated, 24 enteric illness-associated, and three other (two unknown and one chemical or toxin) etiologies (Table 7) (Figure 2). Outbreaks resulted in at least 2,140 cases of illness, 563 hospitalizations (26% of cases), and 88 deaths (4% of cases). At least one etiologic agent was identified in 212 (99%) outbreaks (Tables 1, 2, 3, 4, 5, and 6).

Water Systems, Sources, and Contributing Factors

Community or noncommunity water systems (i.e., public) were linked with 172 (80%) outbreaks, 22 (10%) outbreaks with unknown water systems, 17 (8%) with individual or private systems (i.e., unregulated), and two (0.9%) with other systems; one system type (0.5%) was not reported. Water from individual or private water systems was implicated in 944 (44%) cases, 52 (9%) hospitalizations, and 14 (16%) deaths (Tables 1–6). Drinking water systems with groundwater sources accounted for 82 (38%) outbreaks, surface water sources accounted for 57 (27%) outbreaks, and unknown water sources accounted for 61 (29%) outbreaks (Table 7). A total of 454 contributing factors (practices and factors that led to the outbreak) were reported for 144 (67%) outbreaks (Tables 8 and 9). A total of 393 contributing factors were reported for biofilm-associated outbreaks and 61 for enteric illness-associated outbreaks.

Enteric Illness-Associated Etiologies

Outbreaks of enteric illness included 24 (11%) reports implicating Campylobacter (n = 2; 1%), Cryptosporidium (n = 2; 1%), Escherichia coli (n = 1; 0.5%), Giardia (n = 3; 1%), norovirus (n = 7; 3%), Shigella (n = 4; 2%), and multiple etiologies (n = 5; 2%) (Tables 1, 2, 3, 4, 5, and 6). The enteric illness outbreaks resulted in 1,299 (61%) cases, 10 (2%) hospitalizations, and no deaths. Seventeen outbreaks were linked to norovirus, Shigella, Campylobacter, or multiple etiology outbreaks including these three pathogens and were implicated in 1,225 (57%) cases.

Water System and Water Source

The largest number of cases reported for a single outbreak was 693 (32%). This outbreak was linked to water from an individual or private water system that was contaminated with norovirus and enteropathogenic E. coli (Table 4). When water source (e.g., groundwater) was known (n = 14), wells were identified in 13 (93%) of enteric illness outbreaks, regardless of water system (Tables 1, 2, 3, 4, 5, and 6).

Contributing Factors

A total of 61 (13%) contributing factors were reported for enteric illness outbreaks (Table 9). Water source was the most cited contributing factor type for enteric illness outbreaks, described by 34 (56%) individual contributing factors. Contamination through limestone or fissured rock (e.g., karst) (n = 7; 11%), improper construction or location of a well or spring (n = 7; 11%), and flooding or heavy rains (n = 5; 8%) were the most reported source water contributing factors for enteric illness outbreaks. No disinfection (n = 9, 15%), no filtration (n = 5, 8%), and chronically inadequate disinfection (n = 2; 3%) were the most frequently reported treatment contributing factors for enteric disease outbreaks (Table 9).

Biofilm-Associated Etiologies

Biofilm-associated outbreaks comprised 184 Legionella (86%), two NTM (1%), and one Pseudomonas (0.5%) outbreaks (Table 7). Legionella-associated outbreaks generally increased in number over the study period (14 in 2015, 31 in 2016, 30 in 2017, 34 in 2018, 33 in 2019, and 18 in 2020). The Legionella outbreaks resulted in 786 (37%) cases (Table 7), 544 (97%) hospitalizations, and 86 (98%) deaths (Tables 1, 2, 3, 4, 5, and 6).

Water System

Legionella was the most implicated etiology in public water system outbreaks, associated with 160 (92%) outbreaks, 666 (60%) cases, 462 (97%) hospitalizations, and 68 (97%) deaths related to community and noncommunity water systems (Tables 1, 2, 3, 4, 5, and 6). Individual or private water system outbreaks associated with Legionella resulted in 71 (8%) cases, 48 (92%) hospitalizations, and 14 (100%) deaths.

Contributing Factors

A total of 393 (87%) contributing factors were reported for Legionella and other biofilm pathogen-associated outbreaks (Table 8). Premise or point of use was the most cited contributing factor type for all biofilm-associated pathogen outbreaks and was linked with 287 (73%) individual contributing factors. The most reported premise or point of use contributing factors were Legionella species in water system (n = 67; 17%), Legionella growth-promoting water temperatures and permissive chlorine levels within the building potable water system (n = 35; 9%), water temperature ≥86°F (≥30°C) (n = 28; 7%), and aging plumbing components (e.g., pipes, tanks, and valves) (n = 27; 7%) (Table 8).

Water Treatment and Water Treatment Methods

A total of 183 (86%) outbreak reports contained information about water treatment. Among all outbreaks, disinfection was the reported water treatment for 116 (54%) drinking water systems, unknown water treatment for 49 (23%) drinking water systems, and no water treatment for 17 (8%) drinking water systems (Tables 1, 2, 3, 4, 5, and 6). Seventy-nine outbreak reports (37%) indicated that chlorine was the water treatment method (e.g., description), 99 (46%) reported unknown or no treatment description, and 12 (6%) reported chloramine as the treatment description.

Settings

Hospital or health care facility, long-term care facility, and assisted living or rehabilitation facility (i.e., health care) were identified as the exposure settings in 113 (53%) outbreaks, 456 (21%) cases, 372 (66%) hospitalizations, and 75 (87%) deaths (Figure 3). Furthermore, in the health care facility setting, Legionella was implicated in 111 (52%) outbreaks, 444 (21%) cases, 364 (65%) hospitalizations, and 73 (85%) deaths. Hotels, motels, lodges, or inns were implicated in 35 (16%) outbreaks, 225 (11%) cases, 85 (15%) hospitalizations, and three (3%) deaths, all of which were caused by Legionella (Tables 1, 2, 3, 4, 5, and 6). Finally, Legionella was reported to NORS in 2015 and 2019 as the cause of three outbreaks in private residences resulting in seven (0.3%) cases, four (0.7%) hospitalizations, and no deaths (Tables 1 and 4) (Figure 3) (2).

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Discussion

Drinking water treatment, regulations, and public health programs reduce the risk for exposure to drinking water pathogens, chemicals, and toxins in the United States. Recent estimates of waterborne infectious illness and health care cost effects in the United States have revealed that biofilm-associated pathogens, Legionella and NTM, have emerged as the predominant causes of hospitalizations and deaths from waterborne and drinking water-related disease (3). However, NTM infections are not nationally notifiable diseases and cases and outbreaks might remain undetected (3). Furthermore, during 2015–2020, Legionella-associated outbreaks continued to increase and were the leading cause of nationally reported drinking water-related outbreaks, hospitalizations, and deaths. This trend was primarily influenced by the increasing number and proportion of Legionella-associated outbreaks linked with community and noncommunity water systems (Figures 4 and 5) (6). In addition, Legionella was implicated in all lodging and nearly all (n = 111; 98%) health care-associated biofilm-related outbreaks. Furthermore, Legionella-associated outbreaks in health care settings resulted in approximately two thirds (n = 364; 65%) of hospitalizations and three fourths (n = 73; 85%) of deaths reported during this period. These findings highlight the severity of Legionella infection in the health care setting (13). Legionella also was reported for the first time to NORS as the cause of three outbreaks in private residences. Legionella-associated outbreaks in private residences is an emerging concern. Additional data are needed to better characterize the role of premise plumbing systems in private homes as a potential source of exposure to Legionella and Legionnaires’ disease outbreaks (14). These outbreaks illustrate the importance of effective regulations, water management programs, and public health prevention programs that include communications to reduce the risk for biofilm pathogen growth and spread in public drinking water systems, building water systems, and private homes (4,15,16).

Enteric illness outbreaks represent 11% (n = 24) of the outbreaks, approximately 60% of the cases during this reporting period. Settings varied widely, including mobile home parks, lodging, amusement parks, farms, camps, and private residences. Enteric illness outbreaks associated with norovirus, Shigella, Campylobacter, or multiple etiology outbreaks were primarily associated with individual or private and community water systems. One outbreak of norovirus and enteropathogenic E. coli that resulted in 693 (32%) cases occurred in an amusement park setting because of an overly pumped, improperly constructed well with chronically inadequate disinfection. Wells also were identified as the water source when reported, regardless of water system type (i.e., community or individual or private) in nearly all (n = 13; 93%) enteric illness drinking water outbreaks. No disinfection was reported in nearly 75% (n = 11) of these outbreaks when water treatment was known, underscoring the importance of proper well construction, location (i.e., under the influence of surface water or proximity to wastewater disposal system), operation, and maintenance (17–20).

Understanding and communicating contributing factors related to waterborne outbreaks can lead to improved outbreak prevention, response, and communication practices (21,22). Most drinking water-associated outbreaks have multiple contributing factors, and the most frequently reported types vary between Legionella-associated and enteric illness outbreaks. For example, premise plumbing or point of use is the most cited contributing factor type for Legionella-associated outbreaks, whereas water source is most cited for enteric illness outbreaks. Furthermore, most Legionella-associated outbreak investigations are prompted by cases associated with premise plumbing systems. As a result, premise plumbing contributing factors (e.g., inadequate disinfection or Legionella-promoting water temperatures) are frequently identified. Determining the potential role of other upstream contributing factor types (e.g., water distribution systems) might be difficult. Whereas enteric illness investigations outbreaks frequently result from upstream contributing factors (e.g., disinfection failure or well or groundwater contamination) and can result in many cases of illness. The observed range of biofilm and enteric drinking water pathogen contributing factors illustrates the complexity of drinking water-related disease prevention and the need for water source-to-tap prevention strategies (16,20,23).

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Limitations

The findings in this report are subject to at least four limitations. First, reporting to NORS is voluntary, and surveillance, outbreak investigation, and reporting capabilities vary by jurisdiction. Outbreak surveillance data might not represent the characteristics of all outbreaks and likely underestimate the actual occurrence of outbreaks. Therefore, NORS data should not be used to estimate the actual number of outbreaks. Reports of investigated outbreaks vary, and data are limited to what is available and reported by jurisdictions. Second, Legionella-associated outbreak investigations can continue for years, with new cases of illness occurring after extended periods; jurisdictions might not report to NORS until conclusion of the investigation. Third, the COVID-19 pandemic might have affected jurisdictions’ ability to report waterborne disease outbreaks during 2019–2020. Finally, contributing factors were not available for 70 (33%) drinking water-associated outbreaks, and water treatment was unknown for 49 (23%) drinking water-associated outbreaks. Furthermore, certain Legionella and biofilm-associated pathogen outbreak contributing factors that were self-reported as distribution factor types by users align more closely with premise or point-of-use factor types, possibly resulting in misclassification bias. Legionella species in water systems was frequently reported as a contributing factor and does not provide insight into factors that led to Legionella growth and spread within the water system.

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Future Directions

NORS was updated substantially (CDC 52.14 Form; https://www.cdc.gov/nors/forms.html) in January 2023. The update streamlined the environmental sampling results section, added a section about outbreaks caused by Legionella and other biofilm-associated pathogens, revised the contributing factors section, and created a new interventions section to capture interventions that were recommended or implemented to help stop outbreaks. Future analyses can leverage these NORS updates to improve understanding of biofilm-associated outbreaks, contributing factors, interventions, and water management program failures. Previous studies have highlighted the importance of examining data from Legionella-associated outbreak investigations, including water management program failures which can lead to improved water management practices (13,21,22). Both the Veterans Health Administration and the Centers for Medicare and Medicaid Services have directives requiring the implementation of water management programs in specific health care facilities to reduce the risk related to biofilm-associated pathogens, including Legionella (24,25).

Improvements in biofilm-associated pathogen surveillance and outbreak reporting could lead to greater outbreak detection and guide disease prevention strategies. Recent efforts to estimate the illness and health care cost impacts of waterborne disease in the United States have revealed that Legionella, NTM, and biofilm-associated pathogens have emerged as the predominant causes of hospitalizations and deaths from waterborne diseases, including those linked to drinking water exposures (3). The Council of State and Territorial Epidemiologists has a standardized case definition for extrapulmonary NTM infections (opportunistic infections of wounds, soft tissue, or joints) to ensure consistency in reporting, and to identify outbreaks (26). NTM case reporting, and definitions vary across state and local public health departments, with certain departments reporting pulmonary and extrapulmonary, extrapulmonary only, or NTM site and species not specified (27–29). Health departments could consider making extrapulmonary NTM infections reportable within their jurisdictions (26). In addition, active population-based NTM surveillance, currently occurring in certain jurisdictions, will provide important data for monitoring the illness and health care cost impacts of disease, identifying affected populations, and informing public health prevention strategies (26,28).

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Conclusion

Public health surveillance is essential to monitor trends in waterborne disease and detect outbreaks related to drinking water exposures. During 2015–2020, public health officials from 28 states reported 214 outbreaks associated with drinking water. These outbreaks resulted in at least 2,140 cases of illness, 563 hospitalizations (26% of cases), and 88 deaths (4% of cases). Legionella-associated outbreaks increased in number and were the leading cause of drinking water-associated outbreaks reported to NORS during the surveillance period, including hospitalizations and deaths. Primary prevention of Legionella-associated outbreaks through biofilm control and water management remains critical in health care and nonhealth care settings. Outbreaks of enteric illness primarily linked to wells represented over half of the cases during the reporting period, underscoring the importance of disease prevention efforts related to groundwater. The emergence of biofilm-associated pathogens as the primary influence of drinking water-associated outbreaks, along with the risk for enteric illness outbreaks capable of causing large numbers of cases, highlights the need for agile waterborne disease surveillance, prevention, and outbreak response programs. Drinking water source-to-tap partnership and prevention strategies are critical to addressing the emerging issue of biofilm-associated disease and to guide holistic biofilm pathogen prevention strategies. Drinking water regulations and water management programs are essential to controlling pathogens in drinking water to prevent drinking water-associated outbreaks.

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Acknowledgments

Amy Freeland, Vincent Hill, Jonathan Yoder, National Center for Emerging and Zoonotic Infectious Diseases, CDC; state, territory, and local waterborne disease investigators, epidemiologists, and environmental health personnel.

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Corresponding author: Jasen Kunz, Division of Foodborne, Waterborne, and Environmental Diseases, National Center for Emerging and Zoonotic Infectious Diseases, CDC. Telephone: 770-488-7056; Email: izk0@cdc.gov.

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1Division of Foodborne, Waterborne, and Environmental Diseases, National Center for Emerging and Zoonotic Infectious Diseases, CDC; 2Chenega Corporation, Atlanta, Georgia; 3Division of Bacterial Diseases, National Center for Immunization and Respiratory Diseases, CDC

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Conflict of Interest

All authors have completed and submitted the International Committee of Medical Journal Editors form for disclosure of potential conflicts of interest. No potential conflicts of interest were disclosed.

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  20. Wallender EK, Ailes EC, Yoder JS, Roberts VA, Brunkard JM. Contributing factors to disease outbreaks associated with untreated groundwater. Ground Water 2014;52:886–97. https://doi.org/10.1111/gwat.12121 PMID:24116713
  21. Clopper BR, Kunz JM, Salandy SW, Smith JC, Hubbard BC, Sarisky JP. A methodology for classifying root causes of outbreaks of Legionnaires’ disease: deficiencies in environmental control and water management. Microorganisms 2021;9:89. https://doi.org/10.3390/microorganisms9010089 PMID:33401429
  22. Garrison LE, Kunz JM, Cooley LA, et al. Deficiencies in environmental control identified in outbreaks of Legionnaires’ disease—North America, 2000–2014. MMWR Morb Mortal Wkly Rep 2016;65:576–84. https://doi.org/10.15585/mmwr.mm6522e1 PMID:27281485
  23. Donohue MJ, Vesper S, Mistry J, Donohue JM. Impact of chlorine and chloramine on the detection and quantification of Legionella pneumophila and Mycobacterium species. Appl Environ Microbiol 2019;85:e01942-19. https://doi.org/10.1128/AEM.01942-19 PMID:31604766
  24. Centers for Medicare & Medicaid Services. Requirement to reduce Legionella risk in healthcare facility water systems to prevent cases and outbreaks of Legionnaires’ disease. Baltimore, MD: Centers for Medicare & Medicaid Services; 2017. https://www.cms.gov/Medicare/Provider-Enrollment-and-Certification/SurveyCertificationGenInfo/Downloads/QSO17-30-HospitalCAH-NH-REVISED-.pdf
  25. Department of Veterans Affairs. Prevention of healthcare-associated Legionella disease and scald injury from water systems. Washington, DC: US Department of Veterans Affairs; 2022. https://www.va.gov/vhapublications/publications.cfm?Pub=1
  26. CDC. Healthcare-associated infections (HAIs). Atlanta, GA: US Department of Health and Human Services, CDC; 2023. https://www.cdc.gov/hai/organisms/ntm/health-departments.html
  27. Shih DC, Cassidy PM, Perkins KM, Crist MB, Cieslak PR, Leman RL. Extrapulmonary nontuberculous mycobacterial disease surveillance—Oregon, 2014–2016. MMWR Morb Mortal Wkly Rep 2018;67:854–7. https://doi.org/10.15585/mmwr.mm6731a3 PMID:30091968
  28. Grigg C, Jackson KA, Barter D, et al. Epidemiology of pulmonary and extrapulmonary nontuberculous mycobacteria infections at 4 US emerging infections program sites: a 6-month pilot. Clin Infect Dis 2023;77:629–37. https://doi.org/10.1093/cid/ciad214 PMID:37083882
  29. Mercaldo RA, Marshall JE, Cangelosi GA, et al. Environmental risk of nontuberculous mycobacterial infection: strategies for advancing methodology. Tuberculosis (Edinb) 2023;139:102305. https://doi.org/10.1016/j.tube.2023.102305 PMID:36706504

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TABLE 1. Waterborne disease outbreaks associated with drinking water,* by state and month of first case onset — Waterborne Disease and Outbreak Surveillance System, United States, 2015Return to your place in the text
State Month Etiology† No. of cases No. of hospitalizations No. of deaths Type of water system§ Water source description Treatment Treatment description Water setting
Arizona October Campylobacter jejuni (S), norovirus (S), Shigella sonnei subgroup D (S) 250 0 0 Community Well No treatment Not reported Community or municipality
Florida February Legionella sp. 3 1 0 Community Unknown Disinfection Unknown Private residence
Florida June L. pneumophila serogroup 1 7 7 2 Community Unknown Disinfection Chlorine dioxide Assisted living or rehabilitation facility
Florida September L. pneumophila serogroup 1 2 2 0 Community Well Disinfection Chloramine Hotel, motel, lodge, or inn
Florida November L. pneumophila 5 4 0 Community Unknown Filtration – treatment plant Unknown Assisted living or rehabilitation facility
Florida November L. pneumophila 2 2 1 Community Well Disinfection Chlorine Hospital or health care facility
Florida December L. pneumophila serogroup 1 2 2 0 Community Well Disinfection Not reported Hotel, motel, lodge, or inn
Georgia July L. pneumophila serogroup 1 3 3 0 Community Not reported Disinfection Unknown Hospital or health care facility
Georgia December L. pneumophila 4 4 1 Community Not reported Disinfection Chlorine Hospital or health care facility
Georgia December L. pneumophila serogroup 1 (S) 6 6 0 Community Not reported Disinfection Chlorine Factory or industrial facility
Illinois July L. pneumophila serogroup 1 58 36 13 Individual or private River or stream Disinfection Unknown Long-term care facility
Illinois October L. pneumophila serogroup 1 2 1 0 Unknown Not reported Unknown Not reported Long-term care facility
Maryland October L. pneumophila serogroup 1 4 1 0 Community Well Unknown Not reported Hotel, motel, lodge, or inn
Michigan May L. pneumophila serogroup 1 43 43 5 Community River or stream Disinfection Chlorine Hospital or health care facility
Minnesota December L. pneumophila serogroup 1 2 2 0 Community Unknown No treatment Not reported Restaurant or cafeteria
Missouri March L. pneumophila serogroup 1 3 0 1 Not reported Not reported Not reported Not reported Not reported
New York January L. pneumophila serogroups 1 and 6 4 4 1 Community Lake, reservoir, or impoundment Disinfection Chlorine, ultraviolet light Hospital or health care facility
New York March L. pneumophila serogroups 1 and 4 5 3 1 Community Lake, reservoir, or impoundment Disinfection, other chemical Chlorine, not reported Hospital or health care facility
New York March Shiga-toxin producing Escherichia coli 6 0 0 Individual or private Not reported Unknown Not reported Farm or agricultural setting
New York December Giardia duodenalis 3 0 0 Individual or private Well No treatment Not reported Private residence
Ohio January Nontuberculosis mycobacteria (S) 4 0 0 Unknown Not reported Not reported Not reported Hospital or health care facility
Pennsylvania July L. pneumophila 2 2 0 Individual or private Well Disinfection Chlorine Casino
Utah June Campylobacter jejuni, Giardia (S) 9 1 0 Community Well No treatment, disinfection Not reported, other Private residence

Abbreviation: S = suspected.
* N = 23 outbreaks.
† Etiologies listed are confirmed unless indicated as S. For multiple-etiology outbreaks, etiologies are listed in alphabetical order.
§ Community and noncommunity water systems are defined by the Environmental Protection Agency as public water systems that have ≥15 service connections or serve an average of ≥25 residents for ≥60 days per year. A community water system serves year-round residents of a community, subdivision, or mobile home park. A noncommunity water system serves an institution, industry, camp, park, hotel, or business and can be nontransient or transient. Nontransient systems serve ≥25 of the same persons for ≥6 months of the year but not year round (e.g., factories and schools), whereas transient systems provide water to places in which persons do not remain for long periods (e.g., restaurants, highway rest stations, and parks). Individual water systems are small systems not owned or operated by a water utility that have <15 connections or serve <25 persons.
Two community water systems were reported for this outbreak.

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TABLE 2. Waterborne disease outbreaks associated with drinking water,* by state and month of first case onset — Waterborne Disease and Outbreak Surveillance System, United States, 2016Return to your place in the text
State Month Etiology† No. of cases No. of hospitalizations No. of deaths Type of water system§ Water source description Treatment Treatment description Water setting
Arizona August Norovirus GI.GI.3 32 0 0 Individual or private Not reported Unknown Not reported Hotel, motel, lodge, or inn
Colorado September Legionella sp., L. pneumophila serogroup 1 (S) 2 2 0 Community Lake, reservoir, or impoundment Disinfection Chlorine Assisted living or rehabilitation facility
Florida March L. pneumophila serogroup 1 (S) 2 1 0 Individual or private Well Disinfection Chlorine Camp or cabin
Florida March L. pneumophila serogroup 1 4 2 0 Community Unknown Disinfection Chlorine Assisted living or rehabilitation facility
Florida March L. pneumophila serogroup 1 2 2 0 Community Well Disinfection Chlorine Hospital or health care facility
Florida May L. pneumophila serogroup 1 4 1 0 Community Unknown Disinfection Chlorine Assisted living or rehabilitation facility
Florida June L. pneumophila serogroup 1 6 6 0 Community Unknown Disinfection Chlorine Assisted living or rehabilitation facility
Florida June L. pneumophila serogroup 1 3 0 0 Community Well Disinfection Unknown Hotel, motel, lodge, or inn
Florida September L. pneumophila serogroup 1 2 2 0 Community Well Disinfection Chloramine Hotel, motel, lodge, or inn
Georgia January L. pneumophila serogroup 1 8 8 1 Community Not reported Disinfection Chlorine Hospital or health care facility
Georgia May L. pneumophila serogroup 1 2 2 0 Community Not reported Disinfection Chlorine dioxide Hospital or health care facility
Georgia July L. pneumophila serogroup 1 (S) 2 2 0 Community Not reported Disinfection Chlorine Hospital or health care facility
Georgia October L. pneumophila, L. pneumophila serogroup 1 2 1 0 Community Not reported Disinfection Chlorine Hotel, motel, lodge, or inn
Georgia November L. pneumophila serogroup 1 3 3 2 Community Not reported Disinfection Chlorine Hospital or health care facility
Hawaii April Legionella, L. pneumophila (S), L. pneumophila serogroup 1 (S) 2 2 0 Community Unknown Disinfection Chlorine Apartment or condominium
Illinois February L. pneumophila serogroup 1 5 2 0 Community Not reported Unknown Not reported Long-term care facility
Illinois May L. pneumophila serogroup 1 2 1 0 Community Not reported Disinfection Chlorine Casino
Illinois July L. pneumophila serogroup 1 6 2 0 Community Not reported Unknown Not reported Long-term care facility
Illinois September L. pneumophila serogroup 1 2 2 1 Community Not reported Unknown Not reported Hotel, motel, lodge, or inn
Maryland March Pseudomonas aeruginosa 8 8 2 Community River or stream Coagulation, flocculation, settling or sedimentation, filtration (treatment plant), disinfection, other chemical Unknown, chlorine Hospital or health care facility
Maryland April L. pneumophila serogroup 1 2 2 1 Community Lake, reservoir, or impoundment Coagulation, disinfection, flocculation, filtration (treatment plant), settling or sedimentation, other chemical Chlorine, rapid sand Other
Maryland May L. pneumophila serogroup 1 3 3 0 Community Lake, reservoir, or impoundment Coagulation, disinfection, flocculation, filtration (treatment plant), settling or sedimentation, other chemical Chlorine, rapid sand Other
Maryland July L. pneumophila serogroup 1 2 2 0 Community River or stream Coagulation, flocculation, settling or sedimentation, filtration (treatment plant), disinfection, other chemical Unknown, chlorine Hotel, motel, lodge, or inn
Maryland July L. pneumophila serogroup 1 3 1 0 Community Lake, reservoir, or impoundment Coagulation, disinfection, flocculation, filtration (treatment plant), settling or sedimentation, other chemical Chlorine, rapid sand Long-term care facility
Michigan August L. pneumophila serogroup 1 3 3 0 Community River or stream Disinfection Chloramine Hospital or health care facility
Missouri June Giardia duodenalis (S), Shiga-toxin producing E. coli O157:H7 (S), Shiga-toxin producing Escherichia coli O111 (S) 13 1 0 Community Well Disinfection Chlorine Camp or cabin
New York October L. pneumophila serogroup 1 (S) 4 2 0 Nontransient, noncommunity Well Disinfection, softening Chlorine Hospital or health care facility
Ohio March L. pneumophila serogroup 1 5 5 2 Community Lake, reservoir, or impoundment Disinfection Chlorine Apartment or condominium
Ohio November L. pneumophila serogroup 1 2 2 1 Community Well Disinfection Chlorine Long-term care facility
Oklahoma October Nontuberculosis mycobacteria 11 0 0 Community Lake, reservoir, or impoundment Unknown Not reported Unknown
Pennsylvania February L. pneumophila serogroup 1 4 3 1 Community River or stream Disinfection Copper-silver ionization Hospital or health care facility
Pennsylvania May L. pneumophila serogroup 1 3 3 1 Community River or stream Disinfection Copper-silver ionization Hospital or health care facility
Pennsylvania May L. pneumophila serogroup 1 6 3 1 Unknown Not reported Unknown Not reported Assisted living or rehabilitation facility
Pennsylvania August L. pneumophila 4 4 0 Community River or stream Unknown Not reported Other
Pennsylvania October L. pneumophila serogroup 1 3 2 0 Unknown Not reported Unknown Not reported Long-term care facility
South Carolina November L. pneumophila serogroup 1 4 3 0 Community Unknown Unknown Not reported Hotel, motel, lodge, or inn
Tennessee July L. pneumophila serogroup 1 42 24 0 Community Unknown Disinfection Chlorine Hotel, motel, lodge, or inn
Texas December L. pneumophila serogroup 1 4 4 1 Unknown Not reported Unknown Not reported Unknown
Virginia October Norovirus (S) 23 0 0 Individual or private Well No treatment Not reported Hall or meeting facility
Virginia November Norovirus GI 14 0 0 Individual or private Well No treatment Not reported Hall or meeting facility
Virginia November Norovirus GI.GI.3 31 1 0 Individual or private Well No treatment Not reported Hall or meeting facility
Virginia November Norovirus (S) 10 0 0 Individual or private Well No treatment Not reported Hall or meeting facility
Virginia December Norovirus GI.GI.3 25 0 0 Individual or private Well No treatment Not reported Hall or meeting facility
Washington July L. pneumophila serogroup 1 5 5 2 Community Not reported Unknown Not reported Hospital or health care facility

Abbreviation: S = suspected.
* N = 44 outbreaks.
† Etiologies listed are confirmed unless indicated as S. For multiple-etiology outbreaks, etiologies are listed in alphabetical order.
§ Community and noncommunity water systems are defined by Environmental Protection Agency as public water systems that have ≥15 service connections or serve an average of ≥25 residents for ≥60 days per year. A community water system serves year-round residents of a community, subdivision, or mobile home park. A noncommunity water system serves an institution, industry, camp, park, hotel, or business and can be nontransient or transient. Nontransient systems serve ≥25 of the same persons for ≥6 months of the year but not year round (e.g., factories and schools), whereas transient systems provide water to places in which persons do not remain for long periods of time (e.g., restaurants, highway rest stations, and parks). Individual water systems are small systems not owned or operated by a water utility that have <15 connections or serve <25 persons.

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TABLE 3. Waterborne disease outbreaks associated with drinking water,* by state and month of first case onset — Waterborne Disease and Outbreak Surveillance System, United States, 2017Return to your place in the text
State Month Etiology† No. of cases No. of hospitalizations No. of deaths Type of water system§ Water source description Treatment Treatment description Water setting
Connecticut March Legionella pneumophila serogroup 1 3 1 0 Unknown Not reported Not reported Not reported Long-term care facility
Connecticut March L. pneumophila serogroup 1 2 2 1 Unknown Not reported Not reported Not reported Long-term care facility
Florida January L. pneumophila serogroup 1 3 2 0 Community Unknown Disinfection Chlorine Assisted living or rehabilitation facility
Florida February L. pneumophila serogroup 1 2 2 0 Community Not reported Disinfection Unknown Club (requires membership)
Florida April L. pneumophila serogroup 1 3 3 0 Community Well Disinfection Chlorine Community or municipality
Florida May L. pneumophila serogroup 1 2 2 0 Community Not reported Disinfection Unknown Club (requires membership)
Florida May L. pneumophila serogroup 1 2 2 0 Community Not reported Disinfection Unknown Hotel, motel, lodge, or inn
Florida September L. pneumophila serogroup 1 (S) 2 2 1 Community Well Disinfection Chloramine Hospital or health care facility
Florida October L. pneumophila serogroup 1 2 2 0 Other Unknown Disinfection Chloramine Resort
Florida December L. pneumophila serogroup 1 2 2 0 Other (hospital premise plumbing) Not reported Unknown Not reported Hospital or health care facility
Georgia March L. pneumophila serogroup 1 3 3 1 Community Not reported Disinfection Unknown Hospital or health care facility
Georgia March L. pneumophila serogroup 1 2 2 1 Community Not reported Disinfection Unknown Hospital or health care facility
Georgia August L. pneumophila, L. pneumophila serogroup 1 4 4 0 Community Not reported Disinfection Chlorine Hotel, motel, lodge, or inn
Illinois March L. pneumophila serogroup 1 6 4 1 Community River or stream and well Disinfection Unknown Long-term care facility
Illinois May L. anisa, L. pneumophila serogroup 1 2 2 1 Community Lake, reservoir, or impoundment Other chemical Not reported Hospital or health care facility
Maryland July L. pneumophila serogroups 1 and 5 7 6 1 Community River or stream Coagulation, flocculation, settling or sedimentation, filtration (treatment plant), disinfection, other chemical Unknown, chlorine Long-term care facility
Massachusetts May Legionella sp. 2 2 0 Community Not reported Unknown Not reported Hotel, motel, lodge, or inn
Michigan June L. pneumophila serogroup 1, L. pneumophila serogroups 2–14 (S) 4 4 0 Community Other Disinfection Chlorine Hospital or health care facility
Michigan September L. pneumophila serogroup 1 (S) 2 2 0 Community Lake, reservoir, or impoundment Disinfection Chlorine Hotel, motel, lodge, or inn
Minnesota January Copper 5 0 0 Community River or Stream Disinfection Unknown School, college, or university
Nebraska February Campylobacter jejuni 39 3 0 Community Well No treatment Not reported Unknown
Nevada January L. pneumophila serogroup 1 3 0 0 Community Lake, reservoir or impoundment Filtration (treatment plant) Unknown Hotel, motel, lodge, or inn
Nevada January L. pneumophila serogroup 1 2 0 0 Community Lake, reservoir or impoundment Filtration (treatment plant) Unknown Hotel, motel, lodge, or inn
Nevada March L. pneumophila serogroup 1 107 19 0 Community Lake, reservoir or impoundment Filtration (treatment plant) Unknown Hotel, motel, lodge, or inn
Nevada June L. pneumophila serogroup 1 (S) 2 0 0 Community Lake, reservoir or impoundment Filtration (treatment plant) Unknown Hotel, motel, lodge, or inn
Nevada July L. pneumophila serogroup 1 2 0 0 Community Lake, reservoir or impoundment Filtration (treatment plant) Unknown Hotel, motel, lodge, or inn
New York March L. pneumophila serogroup 1 2 2 0 Community Lake, reservoir or impoundment Coagulation, filtration (treatment plant) Activated carbon, rapid sand Hospital or health care facility
New York July L. pneumophila serogroup 1 4 4 0 Community Lake, reservoir or impoundment Disinfection, coagulation, filtration (treatment plant) Chlorine, ultraviolet light Hospital or health care facility
New York October L. pneumophila serogroup 1 2 2 0 Community Well Disinfection Chlorine Hospital or health care facility
North Carolina March L. pneumophila serogroup 1 4 4 0 Community Not reported Unknown Not reported Other
North Carolina July L. pneumophila serogroup 1 2 2 1 Community Not reported Unknown Not reported Long-term care facility
North Carolina September L. pneumophila serogroup 1 3 2 1 Community Not reported Unknown Not reported Long-term care facility
Ohio May L. pneumophila serogroup 1 5 5 1 Individual or private Lake, reservoir, or impoundment Disinfection Chlorine Hospital or health care facility
Ohio July L. pneumophila serogroup 1 2 2 0 Community Lake, reservoir, or impoundment Filtration (treatment plant) Unknown Long-term care facility
Pennsylvania January L. pneumophila 2 1 0 Community Not reported Aeration Not reported Hotel, motel, lodge, or inn
Wisconsin April Legionella, L. pneumophila, L. pneumophila serogroup 1 (S) 2 0 0 Transient, noncommunity Well No treatment Not reported Hotel, motel, lodge, or inn
Wisconsin July L. pneumophila serogroup 1 2 2 0 Community Well No treatment Not reported Long-term care facility
Wisconsin September Giardia duodenalis 6 0 0 Individual or private Well No treatment Not reported Private residence

Abbreviation: S = suspected.
* N = 38 outbreaks.
† Etiologies are confirmed unless indicated as S.
§ Community and noncommunity water systems are defined by the Environmental Protection Agency as public water systems that have ≥15 service connections or serve an average of ≥25 residents for ≥60 days per year. A community water system serves year-round residents of a community, subdivision, or mobile home park. A noncommunity water system serves an institution, industry, camp, park, hotel, or business and can be nontransient or transient. Nontransient systems serve ≥25 of the same persons for ≥6 months of the year but not year round (e.g., factories and schools), whereas transient systems provide water to places in which persons do not remain for long periods of time (e.g., restaurants, highway rest stations, and parks). Individual water systems are small systems not owned or operated by a water utility that have <15 connections or serve <25 persons.

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TABLE 4. Waterborne disease outbreaks associated with drinking water,* by state and month of first case onset — Waterborne Disease and Outbreak Surveillance System, United States, 2018Return to your place in the text
State Month Etiology† No. of cases No. of hospitalizations No. of deaths Type of water system§ Water source description Treatment Treatment description Water setting
Florida March Legionella pneumophila serogroup 1 4 4 0 Community Well Disinfection Chlorine Long-term care facility
Florida April Legionella sp. (S) 3 3 1 Community Other (aquifer) Disinfection Chlorine Hospital or health care facility
Florida April Norovirus 21 0 0 Community Well Disinfection Chlorine Mobile home park
Florida June L. pneumophila serogroup 1 4 1 0 Community Other Disinfection Chlorine Hotel, motel, lodge, or inn
Florida September L. pneumophila serogroup 1 2 2 0 Community Well Disinfection Chlorine Hotel, motel, lodge, or inn
Florida October L. pneumophila serogroup 1 3 3 2 Community Unknown Disinfection Chlorine Long-term care facility
Florida October L. pneumophila (S) 2 2 0 Community Other (aquifer) Disinfection Chlorine Long-term care facility
Florida November L. pneumophila serogroup 1 3 2 0 Community Not reported Disinfection Chlorine Hotel, motel, lodge, or inn
Florida November L. pneumophila serogroup 1 (S) 2 2 1 Community Other (aquifer) Disinfection Chlorine Long-term care facility
Florida December L. pneumophila serogroup 1 2 2 0 Community Not reported Disinfection Chlorine Hotel, motel, lodge, or inn
Georgia October L. pneumophila serogroup 1 2 2 0 Community Not reported Disinfection Chlorine Hospital or health care facility
Idaho July Campylobacter sp. 24 0 0 Community Other Disinfection Not reported Subdivision or neighborhood
Illinois February L. pneumophila serogroup 1 4 2 0 Community River or stream Disinfection Chlorine dioxide Long-term care facility
Illinois September L. pneumophila serogroup 1 3 3 0 Community Well Disinfection Chlorine Other (grocery store)
Illinois September L. pneumophila serogroup 1 (S) 5 5 0 Community Lake, reservoir, or impoundment Disinfection Chlorine Hospital or health care facility
Illinois November L. pneumophila serogroup 1 2 2 0 Unknown Not reported Unknown Not reported Unknown
Illinois November L. pneumophila serogroup 1 3 2 1 Community Well Disinfection Chlorine Apartment or condominium
Illinois December L. pneumophila serogroup 1 (S) 2 2 0 Unknown Not reported Unknown Not reported Unknown
Illinois December L. pneumophila serogroup 1 (S) 2 2 0 Community Lake, reservoir, or impoundment Disinfection Chlorine Hospital or health care facility
Kansas March L. pneumophila (S) 2 0 0 Community River or stream Disinfection Other Hotel, motel, lodge, or inn
Maryland January L. pneumophila serogroup 1 2 2 0 Community Lake, reservoir, or impoundment Coagulation, disinfection, flocculation, filtration (treatment plant), settling or sedimentation, other chemical Chlorine, rapid sand Long-term care facility
Maryland June L. pneumophila serogroup 1 3 1 0 Community Well Unknown Not reported Apartment or condominium
Maryland August L. pneumophila 5 5 0 Community Lake, reservoir, or impoundment Coagulation, disinfection, flocculation, filtration (treatment plant), settling or sedimentation, other chemical Chlorine, rapid sand Apartment or condominium
Maryland August L. pneumophila (S) 2 2 0 Community Lake, reservoir, or impoundment Coagulation, disinfection, flocculation, filtration (treatment plant), settling or sedimentation, other chemical Chlorine, rapid sand Other, Other (shelter)
Maryland September L. pneumophila serogroup 1 3 1 0 Community Well Unknown Not reported Apartment or condominium
Maryland September L. pneumophila 3 3 0 Community Lake, reservoir, or impoundment Coagulation, disinfection, flocculation, filtration (treatment plant), settling or sedimentation, other chemical Chlorine, rapid sand Apartment or condominium
Maryland September L. pneumophila serogroup 1 3 1 1 Community Well Unknown Not reported Hotel, motel, lodge, or inn
Massachusetts February Legionella 2 2 0 Community Not reported Unknown Not reported Long-term care facility
Massachusetts April L. pneumophila serogroup 1 2 1 0 Unknown Not reported Unknown Not reported Long-term care facility
Massachusetts September L. pneumophila serogroup 1 2 2 0 Individual or private Not reported Unknown Not reported Private residence
Michigan November L. pneumophila serogroup 1 4 4 3 Community River or stream Disinfection Chlorine Hospital or health care facility
New York July L. pneumophila serogroup 1 (S) 2 0 0 Community Lake, reservoir, or impoundment Disinfection, coagulation, filtration (treatment plant), other (copper silver ionization) Chlorine, ultraviolet light Hospital or health care facility
Ohio June L. pneumophila serogroup 1 6 5 0 Community Lake, reservoir, or impoundment Coagulation Not reported Other
Rhode Island July L. pneumophila serogroup 1 2 1 1 Community Lake, reservoir, or impoundment Disinfection Chlorine Long-term care facility
Rhode Island August L. pneumophila serogroup 1 3 2 2 Community Lake, reservoir, or impoundment Disinfection Chlorine Long-term care facility
South Carolina February L. pneumophila serogroup 1 2 0 0 Community Unknown Disinfection Unknown Hotel, motel, lodge, or inn
Tennessee June Cryptosporidium sp., EAEC, EIEC, EPEC, Giardia sp., Norovirus (multiple genogroups) 693 3 0 Individual or private Well Filtration (home or point of use) Activated carbon Park (amusement)
Texas June Shigella sp. 10 1 0 Unknown Not reported Unknown Not reported Unknown
Texas September Not reported 3 1 0 Community or commercially bottled Not reported Not reported Not reported Not reported
Utah April L. pneumophila serogroup 1 3 0 1 Community Not reported Unknown Not reported Assisted living or rehabilitation facility
Wisconsin January L. pneumophila serogroup 1 (S) 2 2 2 Community Lake, reservoir, or impoundment Disinfection Chlorine Long-term care facility
Wisconsin June L. pneumophila serogroup 1 2 2 1 Community Unknown No treatment Not reported Hotel, motel, lodge, or inn
Wisconsin July L. pneumophila serogroup 1 2 2 0 Community Well Disinfection Chlorine Long-term care facility
Wisconsin November L. pneumophila serogroup 1 14 12 2 Community Unknown Disinfection Chlorine Hospital or health care facility

Abbreviations: EAEC = enteroaggregative Escherichia coli; EIEC = enteroinvasive Escherichia coli; EPEC = enteropathogenic Escherichia coli; S = suspected.
* N = 44 outbreaks.
† Etiologies listed are confirmed, unless indicated as S. For multiple-etiology outbreaks, etiologies are listed in alphabetical order.
§ Community and noncommunity water systems are defined by the Environmental Protection Agency as public water systems that have ≥15 service connections or serve an average of ≥25 residents for ≥60 days per year. A community water system serves year-round residents of a community, subdivision, or mobile home park. A noncommunity water system serves an institution, industry, camp, park, hotel, or business and can be nontransient or transient. Nontransient systems serve ≥25 of the same persons for ≥6 months of the year but not year round (e.g., factories and schools), whereas transient systems provide water to places in which persons do not remain for long periods of time (e.g., restaurants, highway rest stations, and parks). Individual water systems are small systems not owned or operated by a water utility that have <15 connections or serve <25 persons.
Water system classified by the regulatory authority as a noncommunity water system because of the outbreak investigation.

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TABLE 5. Waterborne disease outbreaks associated with drinking water,* by state and month of first case onset — Waterborne Disease and Outbreak Surveillance System, United States, 2019Return to your place in the text
State Month Etiology† No. of cases No. of hospitalizations No. of deaths Type of water system§ Water source description Treatment Treatment description Water setting
Florida January Legionella pneumophila, L. pneumophila serogroup 1 3 3 0 Community Other (aquifer) Disinfection Chlorine Hospital or health care facility
Florida January L. pneumophila 3 3 0 Community Other (aquifer) Disinfection Chlorine Assisted living or rehabilitation facility
Florida February L. pneumophila serogroup 1 2 2 0 Community Other (aquifer) Disinfection Chlorine Long-term care facility
Florida May Legionella (S), L. pneumophila serogroup 1 2 1 0 Community Unknown Disinfection Unknown Long-term care facility
Florida June L. pneumophila 2 1 0 Community Other (aquifer) Disinfection Chlorine Long-term care facility
Florida June Legionella sp. 4 4 2 Community Unknown Disinfection Chloramine Long-term care facility
Florida July L. pneumophila serogroup 1 3 3 1 Community Unknown Disinfection Chlorine Hospital or health care facility
Florida August L. pneumophila, L. pneumophila serogroup 1 (S) 2 2 0 Community Other (aquifer) Disinfection Chlorine Apartment or condominium
Florida August L. pneumophila serogroup 1 (S) 2 2 0 Community Well Disinfection Chloramine Assisted living or rehabilitation facility
Florida September L. pneumophila serogroup 1 5 5 0 Community Other (aquifer) Disinfection Chlorine Hospital or health care facility
Florida September L. pneumophila 2 2 0 Community Other (aquifer) Disinfection Chlorine Long-term care facility
Florida November L. pneumophila serogroup 1 2 1 0 Community Well Disinfection Chloramine Private residence
Florida November L. pneumophila serogroup 1 2 0 0 Community Not reported Disinfection Chlorine Hotel, motel, lodge, or inn
Georgia February L. pneumophila serogroup 1 2 2 0 Community Not reported Unknown Not reported Hospital or health care facility
Illinois January L. pneumophila serogroup 1 2 2 1 Community River or stream Disinfection Chloramine Other (veterans home)
Illinois February L. pneumophila serogroups 1 and 6 3 3 2 Community Lake, reservoir, or impoundment Disinfection Chlorine Hospital or health care facility
Illinois February L. pneumophila serogroup 1 2 2 1 Community Well Disinfection Chlorine Long-term care facility
Illinois May L. pneumophila serogroup 1 2 2 1 Community Lake, reservoir, or impoundment Disinfection Chlorine Hospital or health care facility
Illinois May L. pneumophila serogroup 1 2 2 0 Community Lake, reservoir, or impoundment Disinfection Chlorine Hospital or health care facility
Illinois May L. pneumophila serogroup 1 7 7 1 Community Lake, reservoir, or impoundment Disinfection Chlorine Hospital or health care facility
Illinois May L. pneumophila serogroup 1 3 2 2 Community Lake, reservoir, or impoundment Disinfection Chlorine Apartment or condominium
Illinois June L. pneumophila serogroup 1 2 2 0 Community Lake, reservoir, or impoundment Disinfection Chlorine Hotel, motel, lodge, or inn
Illinois July L. pneumophila serogroup 1 (S) 2 1 0 Community Lake, reservoir, or impoundment Disinfection Chlorine Hotel, motel, lodge, or inn
Illinois October L. pneumophila serogroup 1 2 2 0 Community River or stream Disinfection Chloramine Hospital or health care facility
Illinois December L. pneumophila serogroup 1 2 1 0 Community Lake, reservoir, or impoundment Disinfection Chlorine Long-term care facility
Kentucky November L. pneumophila (S) 8 1 1 Community Not reported Unknown Not reported Long-term care facility
Maryland July L. pneumophila serogroup 1 2 1 0 Community Well Unknown Not reported Apartment or condominium
Massachusetts March L. pneumophila serogroup 1 (S) 2 2 0 Unknown Not reported Unknown Not reported Long-term care facility
Massachusetts September L. pneumophila serogroup 1, L. pneumophila serogroups other than 1–6 4 3 0 Unknown Not reported Unknown Not reported Hospital or health care facility
Massachusetts September L. pneumophila serogroup 1 (S) 2 2 0 Unknown Not reported Unknown Not reported Hospital or health care facility
Massachusetts September L. pneumophila serogroup 1 9 3 0 Community Unknown No treatment Not reported Apartment or condominium
Michigan April L. pneumophila serogroup 1 2 2 0 Community Lake, reservoir, or impoundment Disinfection Chlorine Factory or industrial facility
Michigan June L. pneumophila serogroup 1 (S) 2 2 1 Community Lake, reservoir, or impoundment Disinfection Chlorine Hospital or health care facility
Ohio June Norovirus GII.GII.3[P12], Shiga-toxin producing Escherichia coli O103 30 0 0 Individual or private Not reported No treatment Not reported Hall or meeting facility
Pennsylvania April L. pneumophila serogroup 1 (S) 5 5 0 Community Unknown Unknown Not reported Hospital or health care facility
Pennsylvania July L. pneumophila serogroup 1 (S) 2 2 0 Nontransient, noncommunity Unknown Unknown Not reported Hospital or health care facility
Texas June Shigella sp. 7 0 0 Unknown Not reported Unknown Not reported Unknown
Texas June Cryptosporidium sp. 3 0 0 Unknown Not reported Unknown Not reported Unknown
Texas July Shigella sp. 3 0 0 Unknown Not reported Unknown Not reported Unknown
Texas September Shigella sp. (S) 4 0 0 Unknown Not reported Unknown Not reported Unknown
Virginia July C.hominis 41 0 0 Community Not reported Disinfection Chlorine Military facility
Virginia July L. pneumophila 2 2 0 Unknown Not reported Unknown Not reported Long-term care facility
Virginia November L. pneumophila 3 0 0 Unknown Not reported Unknown Not reported Hospital or health care facility
Wisconsin January L. pneumophila serogroup 1 (S) 2 2 1 Community Lake, reservoir, or impoundment Disinfection Ozone Assisted living or rehabilitation facility
Wisconsin October L. pneumophila serogroup 1 (S) 2 2 0 Individual or private Well No Treatment Not reported Hotel, motel, lodge, or inn

Abbreviation: S = suspected.
* N = 45 outbreaks.
† Etiologies listed are confirmed, indicated as S. For multiple-etiology outbreaks, etiologies are listed in alphabetical order.
§ Community and noncommunity water systems are defined by the Environmental Protection Agency as public water systems that have ≥15 service connections or serve an average of ≥25 residents for ≥60 days per year. A community water system serves year-round residents of a community, subdivision, or mobile home park. A noncommunity water system serves an institution, industry, camp, park, hotel, or business and can be nontransient or transient. Nontransient systems serve ≥25 of the same persons for ≥6 months of the year but not year round (e.g., factories and schools), whereas transient systems provide water to places in which persons do not remain for long periods of time (e.g., restaurants, highway rest stations, and parks). Individual water systems are small systems not owned or operated by a water utility that have <15 connections or serve <25 persons.

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TABLE 6. Waterborne disease outbreaks associated with drinking water,* by state and month of first case onset — Waterborne Disease and Outbreak Surveillance System, United States, 2020Return to your place in the text
State Month Etiology† No. of cases No. of hospitalizations No. of deaths Type of water system§ Water source description Treatment Treatment description Water setting
Florida January Legionella sp. (S) 2 2 0 Community Unknown Disinfection Unknown Hospital or health care facility
Florida January Legionella sp. (S) 2 2 1 Community Unknown Disinfection Chlorine Long-term care facility
Florida January L. pneumophila serogroup 1 2 2 0 Community Not reported Disinfection Chlorine Hospital or health care facility
Florida March Legionella sp. (S) 2 2 0 Community Unknown Disinfection Unknown Long-term care facility
Florida March Legionella sp. 2 2 1 Community Unknown Disinfection Chloramine Assisted living or rehabilitation facility
Florida March Legionella sp. 2 2 0 Community Other (aquifer) Disinfection Chlorine Hospital or health care facility
Florida April L. pneumophila (S) 2 2 0 Community Not reported Disinfection Chlorine Long-term care facility
Florida April L. pneumophila 2 2 1 Community Other (aquifer) Disinfection Chlorine Hospital or health care facility
Florida April Legionella sp. (S) 2 2 0 Community Other (aquifer) Disinfection Unknown Assisted living or rehabilitation facility
Florida September L. pneumophila serogroup 1 (S) 2 2 1 Community Other (aquifer) Disinfection Chlorine Hospital or health care facility
Hawaii March Giardia sp. (S) 2 0 0 Community Unknown Unknown Not reported Unknown
Illinois January L. pneumophila serogroup 1 2 2 0 Community Lake, reservoir, or impoundment Disinfection Chlorine Long-term care facility
Illinois August L. anisa (S), L. pneumophila serogroup 1 (S) 3 3 1 Community Unknown Disinfection Chlorine Apartment or condominium
Illinois September L. pneumophila serogroup 1 2 2 0 Community River or stream Disinfection Chloramine Prison or jail (juvenile or adult)
Maryland July L. pneumophila serogroup 1 2 1 0 Community Well Unknown Not reported Hotel, motel, lodge, or inn
Massachusetts June Unknown 24 0 0 Community Not reported Unknown Not reported Private residence
Massachusetts August L. pneumophila serogroup 1 3 3 0 Unknown Not reported Unknown Not reported Hospital or health care facility
Ohio February L. pneumophila (S) 2 2 0 Community Other (city) Unknown Not reported Hospital or health care facility
Pennsylvania April L. pneumophila serogroup 1 (S) 3 3 0 Community Not reported Unknown Not reported Apartment or condominium
Wisconsin April L. pneumophila serogroup 1 (S) 2 2 0 Community Unknown No treatment Not reported Long-term care facility

Abbreviation: S = suspected.
* N = 20 outbreaks.
† Etiologies listed are confirmed unless indicated as S.
§ Community and noncommunity water systems are defined by the Environmental Protection Agency as public water systems that have ≥15 service connections or serve an average of ≥25 residents for ≥60 days per year. A community water system serves year-round residents of a community, subdivision, or mobile home park. A noncommunity water system serves an institution, industry, camp, park, hotel, or business and can be nontransient or transient. Nontransient systems serve ≥25 of the same persons for ≥6 months of the year but not year round (e.g., factories and schools), whereas transient systems provide water to places in which persons do not remain for long periods of time (e.g., restaurants, highway rest stations, and parks). Individual water systems are small systems not owned or operated by a water utility that have <15 connections or serve <25 persons.

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Return to your place in the textFIGURE 1. Number of reported drinking water-associated outbreaks,* by state of exposure — National Outbreak Reporting System, United States, 2015–2020
Figure is a map illustrating the number of reported drinking water-associated outbreaks, by state of exposure, in the United States during 2015–2020. The data is from the National Outbreak Reporting System.

* N = 214 outbreaks.

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TABLE 7. Rank order (most common to least common) of etiology, water system, water source, and contributing factor types for drinking water-associated outbreaks and associated cases of illness — United States, 2015–2020Return to your place in the text
Characteristic or rank Outbreaks* Cases*
Category No. (%) Category No. (%)
Etiology
1 Bacterium (biofilm associated†), Legionella 184 (86.0) Multiple (enteric)§ 995 (46.5)
2 Virus 7 (3.3) Bacterium (biofilm associated), Legionella 786 (36.7)
3 Bacterium (enteric) 7 (3.3) Virus (enteric) 156 (7.3)
4 Multiple (enteric) 5 (2.3) Bacterium (enteric) 93 (4.3)
5 Parasite (enteric) 5 (2.3) Parasite (enteric) 55 (2.6)
6 Bacterium (biofilm associated), non-Legionella 3 (1.4) Unknown 27 (1.3)
7 Unknown 2 (0.9) Bacterium (biofilm associated), non-Legionella 23 (1.1)
8 Chemical or toxin 1 (0.5) Chemical or toxin 5 (0.2)
Water system
1 Community 169 (79.0) Community 1,106 (51.7)
2 Unknown 22 (10.3) Individual or private 944 (44.1)
3 Individual or private 17 (7.9) Unknown 75 (3.5)
4 Noncommunity 3 (1.4) Noncommunity 8 (0.4)
5 Other 2 (0.9) Other 4 (0.2)
6 Not reported 1 (0.5) Not reported 3 (0.1)
Water source
1 Groundwater 82 (38.3) Groundwater 1,348 (63.0)
2 Unknown 61 (28.5) Surface water 384 (17.9)
3 Surface water 57 (26.6) Unknown 309 (14.4)
4 Mixed 11 (5.1) Mixed 89 (4.2)
5 Not reported 3 (1.4) Not reported 10 (0.5)
Contributing factor type
1 Premise point of use 85 (39.7) Multiple 1,374 (64.2)
2 Not reported 70 (32.7) Premise point of use 389 (18.2)
3 Multiple 46 (21.5) Not reported 238 (11.1)
4 Source 7 (3.3) Source 124 (5.8)
5 Distribution 6 (2.8) Distribution 15 (0.7)

* N = 214 outbreaks; N = 2,140 cases.
† Biofilm-associated drinking water outbreaks in this analysis include outbreaks caused by Legionella (n = 184), nontuberculous Mycobacteria (n = 2), and Pseudomonas (n = 1).
§ Multiple-etiology outbreaks include two enteric bacterial and parasitic; two enteric bacterial and viral; and one enteric bacterial, parasitic, or viral etiologic category.

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Return to your place in the textFIGURE 2. Number of reported drinking water-associated outbreaks,*,† by month of earliest illness onset — National Outbreak Reporting System, United States, 2015–2020
Figure is a histogram showing the number of reported drinking water-associated outbreaks, by month of earliest illness onset, in the United States during 2015–2020. The data is from the National Outbreak Reporting System.

* N = 214 outbreaks.

† Other outbreaks refers to two outbreaks of unknown etiology and one outbreak caused by a chemical or toxin.

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TABLE 8. Rank order (most common to least common) of contributing factors reported for Legionella and other biofilm-associated* drinking water outbreaks† — United States, 2015–2020Return to your place in the text
Type or rank Contributing factor§ No. (%)
Source
1 Unknown 10 (2.5)
2 None 2 (0.5)
3 Use of an alternate source of water by a water utility 1 (0.3)
4 Underchlorinated 1 (0.3)
5 Other 1 (0.3)
Treatment
1 Unknown 8 (2.0)
2 Construction or repair of pipes or components without evidence of contamination 5 (1.3)
3 No disinfection 4 (1.0)
4 Chronically inadequate disinfection 2 (0.5)
5 Contamination during construction or repair of pipes or components 2 (0.5)
6 Change in treatment process 2 (0.5)
7 Temporary interruption of disinfection 1 (0.3)
8 Deficiencies in other treatment processes 1 (0.3)
9 Pipe or component failure or break (e.g., pipes, tanks, or valves) 1 (0.3)
Distribution
1 Legionella-promoting water temperatures and chlorine levels within the potable water system inside building** 15 (3.8)
2 Water temperature ≥30°C (≥86°F) 8 (2.0)
3 Unknown 7 (1.8)
4 Aging water distribution components (e.g., pipes, tanks, or valves) 5 (1.3)
5 Amplification of Legionella pneumophila in colonized municipal water taking place inside hotel** 4 (1.0)
6 Low water pressure or change in water pressure in distribution system 4 (1.0)
7 Lowest hot water temperature documents from 106°F (41.1°C) to 109.5°F (43.1°C) suspected** 4 (1.0)
8 Stagnation of water because of sporadic occupancy** 4 (1.0)
9 Construction or repair of mains without evidence of contamination 3 (0.8)
10 Multiple dead legs (i.e., piping subject to low or no flow because of design or decreased water use) in facility before remediation (i.e., after outbreak)** 2 (0.5)
11 Contamination of mains during construction or repair 2 (0.5)
12 Contamination of storage facility 2 (0.5)
13 Pipe or component failure or break (e.g., pipes, tanks, or valves) 2 (0.5)
14 Cross-connection of potable and nonpotable water pipes resulting in backflow 1 (0.3)
15 Mixing of treated water from different sources 1 (0.3)
16 Corrosion in or leaching from pipes or storage tanks 1 (0.3)
Premise or point of use
1 Legionella species in water system 67 (17.0)
2 Legionella-promoting water temperatures and chlorine levels within potable water system inside building 35 (8.9)
3 Water temperature ≥30°C (≥86°F) 28 (7.1)
4 Aging plumbing components (e.g., pipes, tanks, or valves) 27 (6.9)
5 Deficiency in building or home-specific water treatment after water meter or property line 15 (3.8)
6 Contamination at point of use (tap) 13 (3.3)
7 Legionella-promoting chlorine levels within potable water system inside building 11 (2.8)
8 Temperature control in hot water systems 10 (2.5)
9 Deficiency or contamination of equipment or devices using or distributing water 9 (2.3)
10 Contamination of plumbing during construction or repair 9 (2.3)
11 Stagnation of water because of sporadic occupancy 9 (2.3)
12 Construction or repair of plumbing without evidence of contamination 7 (1.8)
13 Dead-end water lines (i.e., piping subject to low or no flow because of design or decreased water use) 6 (1.5)
14 Unknown 6 (1.5)
15 Low pressure or change in water pressure in plumbing 5 (1.3)
16 Pipe or component failure or break (e.g., pipes, tanks, or valves) 4 (1.0)
17 Hot water temperature >115°F (46.1°C) at point of use 4 (1.0)
18 Amplification of Legionella pneumophila from colonized municipal water in hotel 3 (0.8)
19 Lack of backflow prevention in plumbing 3 (0.8)
20 Lack of adequate flushing following construction 3 (0.8)
21 Hurricane 2 (0.5)
22 Corrosion in or leaching from pipes or storage tanks 2 (0.5)
23 Other 2 (0.5)
24 High- and low-pressure humidification devices 1 (0.3)
25 Contamination at point of use (unknown) 1 (0.3)
26 Improper use of disinfection 1 (0.3)
27 Multiple dead legs (i.e., piping subject to low or no flow because of design or decreased water use) identified in facility 1 (0.3)
28 No supplemental disinfection in facility 1 (0.3)
29 Expansion of plumbing system in phases upon existing infrastructure 1 (0.3)
30 Cross-connection of potable and nonpotable water pipes resulting in backflow 1 (0.3)
Total contributing factors 393 (100.0)

* Biofilm-associated drinking water outbreaks in this analysis include outbreaks caused by Legionella (n = 184), nontuberculous Mycobacteria (n = 2), and Pseudomonas (n = 1).
† N = 187 outbreaks. Outbreaks include those involving community (n = 158), unknown (n = 17), individual or private (n = 6), noncommunity (n = 3), and other (n = 2) water systems.
§ One outbreak might have multiple contributing factors reported.
Percentage is calculated using a denominator of 393 because 393 is the total number of contributing factors reported for the 187 outbreaks.
** Reported as distribution factor types by users but aligns more closely with premise or point-of-use factor types.

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TABLE 9. Rank order (most common to least common) of 61 contributing factors reported for outbreaks of enteric illness associated* with drinking water — United States, 2015–2020Return to your place in the text
Type or rank Contributing factor§ No. (%)
Source
1 Contamination through limestone or fissured rock (e.g., karst) 7 (11.5)
2 Improper construction or location of a well or spring 7 (11.5)
3 Flooding or heavy rains 5 (8.2)
4 Unknown 2 (3.3)
5 Domestic animal contamination (e.g., livestock, concentrated feeding operations, or pets) 2 (3.3)
6 Groundwater under direct influence of surface water (e.g., shallow well) 2 (3.3)
7 Underchlorinated 1 (1.6)
8 Malfunctioning onsite wastewater treatment system 1 (1.6)
9 Irrigation water was cross-connected with drinking water in neighborhood 1 (1.6)
10 Combined sewer overflow 1 (1.6)
11 Pump failure 1 (1.6)
12 Well cap under influence of surface water 1 (1.6)
13 Low water table (e.g., drought or overpumping) 1 (1.6)
14 Fill hose was in direct contact with the ground 1 (1.6)
15 Water system intake failure (e.g., cracked well casing or cracked intake pipe) 1 (1.6)
Treatment
1 No disinfection 9 (14.8)
2 No filtration 5 (8.2)
3 Chronically inadequate disinfection 2 (3.3)
4 Pipe or component failure or break (e.g., pipes, tanks, or valves) 1 (1.6)
5 Inadequate filtration 1 (1.6)
6 Unknown 1 (1.6)
Distribution
1 Cross-connection of potable and nonpotable water pipes resulting in backflow 1 (1.6)
2 Low pressure or change in water pressure in the distribution system 1 (1.6)
3 Unknown 1 (1.6)
Premise or point of use
1 Unknown 2 (3.3)
2 Pump failure 1 (1.6)
3 Cross-connection of potable and nonpotable water pipes resulting in backflow 1 (1.6)
4 Lack of adequate flushing following construction 1 (1.6)
Total 61 (100.0)

* Enteric illness-associated drinking water outbreaks include outbreaks caused by Campylobacter (n = 2), Cryptosporidium (n = 2), Escherichia coli (n = 1), Giardia (n = 3), norovirus (n = 7), Shigella (n = 4), and multiple etiologies (n = 5).
† N = 24 outbreaks. Outbreaks include those involving individual or private (n = 11), community (n = 8), and unknown (n = 5) water systems.
§ One outbreak might have multiple contributing factors reported.
Percentage is calculated using a denominator of 61 because 61 is the total number of contributing factors reported for the 24 outbreaks.

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Return to your place in the textFIGURE 3. Number of reported drinking water-associated outbreaks,* by water setting of exposure†,§ — National Outbreak Reporting System, United States, 2015–2020
Figure is a bar graph illustrating the number of reported drinking water-associated outbreaks, by water setting of exposure, in the United States during 2015–2020. The data is from the National Outbreak Reporting System.

* N = 214 outbreaks.

† Health care setting includes assisted living or rehabilitation facilities, hospital or health care facilities, and long-term care facilities.

§ Other setting includes grocery store, veterans’ home, shelter, and other (not specified).

Other outbreaks refers to two outbreaks of unknown etiology and one outbreak caused by a chemical or toxin.

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Return to your place in the textFIGURE 4. Number of reported drinking water-associated outbreak etiologies,* by Legionella compared with all other etiologies — Waterborne Disease and Outbreak Surveillance System, United States, 2007–2020
Figure is a histogram illustrating the number of reported drinking water-associated etiologies, by Legionella compared with all other etiologies, in the United States during 2007–2020. The data is from the Waterborne Disease and Outbreak Surveillance System.

* N = 366 outbreak etiologies.

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Return to your place in the textFIGURE 5. Number of reported drinking water-associated outbreaks in community and noncommunity water settings,* by Legionella compared with all other etiologies — Waterborne Disease and Outbreak Surveillance System, United States, 2007–2020
Figure is a histogram illustrating the number of reported drinking water-associated outbreak community and noncommunity water settings, by Legionella compared with all other etiologies, in the United States during 2007–2020. The data is from the Waterborne Disease and Outbreak Surveillance System.

* N = 306 water settings.

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Suggested citation for this article: Kunz JM, Lawinger H, Miko S, et al. Surveillance of Waterborne Disease Outbreaks Associated with Drinking Water — United States, 2015–2020. MMWR Surveill Summ 2024;73(No. SS-1):1–23. DOI: http://dx.doi.org/10.15585/mmwr.ss7301a1.

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